Caratterizzazione di complessi metallorganici tramite metodo pump and probe con time delay nel regime dei nanosecondi e dei microsecondi

The aim of the research project of this thesis work is to obtain detailed time-resolved structural information on photodissociation process of organometallic complexes by means of a novel and promising technique, i.e. Time-Resolved Wide Angle X-ray scattering (TR-WAXS). The experimental core of this thesis work consists in a TR-WAXS investigation, using a laser pump/X-ray probe methodology, of the photodissociation reaction of [Ru(bpy)2(py)2]2+ in aqueous solution, performed at the ESRF (beamline ID09B), to which I personally attended. The technique is based on the combination of X-ray ability in probing matter structure at the atomic scale with the temporal resolution obtained using the pump and probe method. The result of such an investigation will be a series of ¿snapshots¿ of the structure of the analyzed system taken at different times delays between an ultra-short laser pump pulse triggering a photodissociation process and the arrive of the X-ray probe pulse (¿molecular movie¿). TR-WAXS investigation of a sample in solution involves some critical issues if compared with X-ray diffraction analysis of crystal systems. In the case of liquid scattering models (e.g. from DFT calculations) are necessarily needed in order to find best match to molecular fingerprints and to correctly interpret experimental data. Accurate data acquisition and treatment is required due to the smallness of effective differential intensity from solute photo-induced structural modification. Moreover, the contribution to differential scattered intensity of solvent response to the heat-release from the solute molecules excited by the laser pulse have to be measured and extracted to overall signal. Although the effort required in interpreting experimental data from an X-ray scattering investigation should be notably harder in the case of liquid-phase, we have to consider that most of biochemical and biological reactions take place in solution: the possibility of analyzing from the structural point of view time evolution of this kind of processes in their naturally complex environment can offer relevant insights towards their complete comprehension. The obtained results from TR-WAXS investigation of the photodissociation reaction of [Ru(bpy)2(py)2]2+ in aqueous solution are relevant: it has been demonstrated that this technique is able to measure, as a function of inter-atomic distance, the release of the pyridine ligand from the metal centre and the subsequent coordination of a water molecule within 300 ns from laser excitation. This investigation has allowed the first observation of a complex fragment (constituted by several low-Z atoms) dissociation resulting in a highly-structured signal in r-space, demonstrating disappearance not only of the first shell ligand of Ru atom, but also of the second and third ones. It's remarkable that the investigated complex [Ru(bpy)2(py)2]2+ can be potentially applied in the field of photoactivale anticancer agents based on organometallic complexes. Light activation offers the advantage of time- and spatial-control of the active species in tissues, potentially reducing undesired secondary effects due to the toxicity of such species. Future advances could allow detailed TR-WAXS studies of the dynamics of photochemical processes under biologically-relevant conditions, providing fundamental insights for the medical application of metallorganic complexes as photoactivable antitumoral drugs